Ball grid arrays (BGA's) formed on multilayer substrates typically include a grid of vias, which are cylindrical structures formed from an electrically conductive material, such as copper, disposed in the substrate in a grid pattern. Typically, the vias extend from one side of the substrate through the various layers therein, to the second side of the substrate. Some of the vias may be connected to ground or a power voltage through one or more of the layers of the substrate, and others of the vias are used to pass electrical signals throughout an attached electrical device. The electrical device, such as an integrated circuit (IC), is attached, through the BGA, to one side of the substrate. The ground and power vias supply ground or power to various pins of the device, and the signal vias interconnect portions of the device to other portions and to other devices on the substrate.
One issue that must be addressed designing and fabricating BGA's is the inductance that is present in the array. Inductance is the ability of a conductor to produce an induced voltage when cut by a magnetic flux. A conductor is a material capable of conveying an electric current. Virtually all conductors have inductance, but the amount of inductance associated with each conductor varies according to a number of factors such as type of conductive material, shape of the conductor, length of the conductor, and so forth. For example, a shorter wire has less inductance than a long wire because less conductor length cut by a magnetic flux produces less voltage. Similarly, a straight wire has less inductance than a coiled wire because the conductor concentrates more conductor length in a given area of flux.
One characteristic of inductors is that the faster the speed at which the flux changes, the more voltage is induced. The flux may take the form of a change in current. For example, alternating current (AC) circuits continually produce an induced voltage because the current is continuously changing. The faster the current changes, the higher the induced voltage, which always opposes the change in current. If current is increased, the polarity of the induced voltage opposes the increase in current, and vice versa. However, it is not necessary for the current to alternate directions. Inductance affects DC circuits whenever the value of the DC current changes, such as when a DC circuit is turned on and off.
There are four types of inductance: system inductance, self-inductance, mutual inductance, and stray inductance. System inductance is a combination of all the self inductances, mutual inductances, and stray inductances found within a circuit. Self inductance is the ability of a conductor to induce voltage in itself when the current changes. Mutual inductance typically occurs whenever two conductors are positioned closely together such that a varying flux resulting from a change in current in Conductor A cuts across and induces voltage in Conductor B. This induced voltage, in turn, generates a magnetic flux that cuts across and induces a voltage in conductor A. Because a current in one conductor can induce voltage in the adjacent conductor, the conductors are said to have mutual inductance. Stray inductance is the inductance of any wiring not included in discrete inductors, for example, traces, capacitors, etc. In most cases, stray inductance is negligible. However, in high frequency circuits, where the current changes very quickly, stray inductance can have appreciable effects. To offset this appreciable effect, traces, leads, and current return path are usually kept as short as possible.
Each of these types of inductance discussed above seriously affects, and in some cases limits, the I/O speeds of integrated circuits. For example, in the case where all the bus outputs of a circuit simultaneously switch the same way, the circuit is deluged with a tidal wave of current. This current surge generates an appreciable induced voltage in the circuit's conductors. The induced voltage flowing opposite to the wave of current, reduces the amount of current flowing through the circuit, thereby slowing the rate of current flow. Furthermore, the induced voltage has other effects on the device signals, such as ground bounce, over/undershoot of the signal waveform and non-monotonic edges on the signal waveform. It is clear that faster I/O times will result if system inductance can be minimized, as well as higher signal integrity.